This invention relates to a measurement signal generating circuit for a linear scale which is adapted to measure relative movement between two objects, and more particularly to a signal processing circuit which is useful to increase an S/N ratio of a Lissajous signal outputted from a photoelectric conversion means.
In a machine tool or the like, accurate measurement of relative movement between a tool and a workpiece is highly important for carrying out precise processing of the workpiece. For this purpose, a variety of measuring devices have been manufactured as commercial products.
One of such measuring devices is practiced in the form of an optical scale utilizing Moire fringes obtained by superposing two optical gratings on each other. The optical scale is generally constructed in such a manner as shown in FIGS. 5(a) and 5(b). More particularly, the optical scale includes a main scale 101 including a transparent glass scale 100 which is formed thereon with gratings (cut lines) so that light-permeable sections and light-impermeable sections are arranged at predetermined pitches. The optical scale also includes an index scale 103 including a transparent glass scale 102 formed thereon with gratings so that light-permeable sections and light-impermeable sections are arranged at predetermined pitches. The main scale 101 and index scale 103, as shown in FIG. 5(a), are arranged opposite to each other at a microinterval. Also, the main scale 101 and index scale 103, as shown in FIG. 5(b), are so arranged that the gratings of the index scale 103 are inclined at a microangle with respect to the gratings of the main scale 101.
The gratings provided on the main scale 101 and index scale 103 are formed at the same pitches by forming chromium on the glass scales 100 and 102 by vacuum deposition and then subjecting it to etching.
Such arrangement of the gratings permits generation of Moire fringes shown in FIG. 6. The Moire fringes are formed at intervals W, so that dark portions or bright portions are obtained at the intervals W. The dark portions or bright portions are downwardly or upwardly moved depending on a direction in which the index scale 103 is laterally moved relative to the main scale 101. In this instance, supposing that pitches of gratings of the main scale 101 and index scale 103 are indicated by P and an inclination angle between both scales 101 and 103 is indicated by xcex8 (rad), the intervals W of the Moire fringes are represented by the following expression:
W=P/xcex8
Thus, the intervals W of the Moire fringes are optically defined to be 1/xcex8 times as large as the grating intervals P. Therefore, when the grating is moved by one pitch P, the Moire fringe is displaced by W, so that movement within the pitch P may be precisely measured by reading a variation in intervals W in a vertical direction.
For example, as shown in FIG. 7, a photoelectric conversion element 110 is provided on the index scale 103 and a light source is provided on a side of the main scale 101 opposite to the photoelectric conversion element 110, so that a variation in current flowing to the photoelectric conversion element 110 may be read while moving the index scale 103 relative to the main scale 101.
More particularly, when a pattern of the Moire fringes is at a state indicated by A in FIG. 7, the amount of light irradiated to the photoelectric conversion element 110 is maximized, so that a current flowing to the photoelectric conversion element 110 reaches a maximum level I1. Then, when the pattern is at a state indicated by B in FIG. 7 due to relative movement between the main scale 101 and the index scale 103, the amount of light irradiated to the photoelectric conversion element 110 is somewhat reduced, so that the current is reduced to a level I2. When the relative movement is further carried out to cause the pattern to be at a state indicated at C, the amount of light irradiated to the photoelectric conversion element 110 is minimized, so that the current is reduced to a minimum level I3. Then, when the index scale 103 is further moved relative to the main scale 101 to cause the pattern to be at a state D in FIG. 7, light irradiated to the photoelectric conversion element 110 is somewhat increased, resulting in the current being increased to a level I2. Moreover, the relative movement is further carried out to cause the pattern to be at a state E in FIG. 7, light irradiated to the photoelectric conversion element 110 is increased to the maximum level again, so that the current may be increased to the maximum level I1.
Thus, the current flowing to the photoelectric conversion element 110 is varied in a manner like a sinusoidal wave, and when the variation elapses by one period, relative movement between the main scale 101 and the index scale 103 is carried out by the grating interval P.
In FIG. 7, only one such photoelectric conversion element 110 is arranged. Alternatively, as shown in FIG. 8, an A phase photoelectric conversion element 111 and a B phase photoelectric conversion element 112 may be arranged while being deviated from each other by one period (interval W) and 90xc2x0. Such arrangement permits a current flowing to the B phase photoelectric conversion element 112 to be deviated by 90xc2x0 with respect to a current flowing to the A phase photoelectric conversion element 111, as shown in FIG. 9. Thus, supposing that a current flowing to the A phase photoelectric conversion element 111 is in the form of a sinusoidal wave, that flowing to the B phase photoelectric conversion element 112 is in the form of cosine wave.
In this instance, a phase of the current flowing to the B phase photoelectric conversion element 112 is advanced or delayed by 90xc2x0 relative to that of the current flowing to the A phase photoelectric conversion element 111 depending on a direction of relative movement between the main scale 101 and the index scale 103. Thus, when the two photoelectric conversion elements 111 and 112 are arranged while being deviated by 90xc2x0 relative to each other, a phase therebetween may be detected, resulting in a direction of the relative movement being detected.
Actually, the conventional optical scale is so constructed that any additional photoelectric conversion element is arranged at a predetermined position, to thereby concurrently output an A phase signal and a B phase signal, as well as inverted A phase and B phase signals respectively obtained by inverting the A phase and B phase signals by 180xc2x0. Such construction permits a DC component to be removed from the signal detected and ensures reliability of the signal and follow-up characteristics at a high speed.
FIG. 4(a) indicates the above-described A phase signal and inverted A phase signal and the above-described B phase signal and inverted B phase signal obtained by arranging four photo-detectors at predetermined positions on the index scale.
Also, FIG. 4(b) shows a circuit for forming a synthesized A-phase signal based on waveforms of the two A phase signals described above, wherein AP and xe2x88x92AP each indicate a photo detector for detecting Moire fringes formed by light permeating between the cut lines of the scale to convert them into an electric signal.
In a sinusoidal current of an inverted phase outputted from each of the photo detectors, one of signals thereof is inverted at a phase thereof through an inversion amplifier A1 and synthesized in an addition circuit ADD constituted by an operational amplifier OP.
Synthesis of the B phase signal is likewise carried out in such a circuit as described above.
Such a circuit structure permits synthesis of a measurement signal from which a DC signal is removed through the single operational amplifier OP, to thereby accomplish a reduction in manufacturing cost. However, it renders adjustment in offset before the synthesis difficult, leading to a deterioration in balance between the A phase signal (B phase signal) and the xe2x88x92A phase signal (xe2x88x92B phase signal).
In view of the above, the conventional optical scale device, as shown in FIG. 4(c), is so constructed that signals outputted from the photo detectors AP and xe2x88x92AP are amplified through current/voltage converters A1 and A3, resulting in individually having a predetermined voltage level. Then, the outputs are numerically added to each other by a differential amplifier OP1, to thereby provide the A phase signal.
Such an approach ensures balance between the A phase signal and the xe2x88x92A phase signal, to thereby prevent generation of strain in a waveform of the A phase signal for measurement synthesized, even when the scale is moved at a high speed.
The conventional optical scale thus constructed is arranged on an NC machine tool to measure relative movement between a tool and a workpiece. In general, the photo detector having the scale mounted thereon to detect the signal is arranged on a side of the machine tool, whereas a signal processing means which carries out processing of the sinusoidal Lissajous signal detected, to thereby display the actual measurement signal is arranged at a different position while being connected to a signal transmission cable.
Also, a linear scale is generally used in a place in which a machine tool or the like is arranged, so that noise generated in the place may be readily picked up by the linear scale, resulting in the noise component being included in a waveform of the signal detected.
When the signal having the noise component included therein is processed to operate a highly accurate measured value of which a phase is divided, a display section of the device indicates a wrong numerical value. Thus, when the linear scale outputs a relative measured value, the device fails to readily correct the error.
In particular, the photo detectors for detecting the four signals constituted by the A phase signal, xe2x88x92A phase signal, B phase signal and xe2x88x92B phase signal are arranged on a support member at predetermined intervals. However, the support member resonates at a natural frequency, to thereby cause a high oscillation frequency to be generated irrespective of each signal phase, when the scale is moved at a high speed or load is applied to the machine tool. When the thus-generated oscillation frequency is introduced as noise into a signal processing circuit, it forms noise at a different phase, which cannot be fully removed even by a signal generating circuit of the differential amplification type which is effective for noise at the same phase.
The present invention has been made in view of the foregoing disadvantage of the prior art.
Accordingly, it is an object of the present invention to provide a measurement signal generating circuit for a linear scale which is capable of reducing an error in measurement by the linear scale.
It is another object of the present invention to provide a measurement signal generating circuit for a linear scale which is capable of effectively removing noise at a different phase as well as that at the same phase.
In accordance with the present invention, a measurement signal generating circuit for a linear scale is provided. The measurement signal generating circuit includes a scale having graduations provided thereon at equal intervals in a direction of movement thereof, a detection means for detecting relative movement of the scale as four measurement signals constituted by a sinusoidal A phase signal, a xe2x88x92A phase signal obtained by inverting the A phase signal, a B phase signal of which a phase is shifted by 90 degrees with respect to the A phase signal and a xe2x88x92B phase signal obtained by inverting the B phase signal, amplification circuits each arranged for amplifying each of the four measurement signals to a predetermined level, a differential amplifier for adding antiphase components outputted from the amplification circuits to each other and outputting them therefrom, and a low-pass filter circuit arranged rearwardly of the differential amplifier to remove a high-frequency noise component.
In a preferred embodiment of the present invention, the low-pass filter circuit is constituted by an active filter.
In a preferred embodiment of the present invention, the low-pass filter circuit is so constructed that a cut-off frequency thereof is varied depending on a speed of movement of the scale.
In a preferred embodiment of the present invention, the low-pass filter circuit uses a switched capacitor for a time constant circuit. The switched capacitor is driven by a clock signal corresponding to a speed of movement of the scale.